1. Field of the Invention
This invention relates to frequency filters. More particularly, the invention is directed to such filters formed by coupling a plurality of electromechanical filters to attain a high frequency filtering capability.
2. Description of the Prior Art
It is well known that electromechanical filters are preferred over filters using discrete resistive, capacitive and inductive components, for many applications. Such electromechanical filters are extremely accurate, repeatable and have inherent frequency stability as to temperature and time.
The instant invention can be readily understood by discussing the principles and concepts as they relate to a particular electromechanical filter -- the Monolithic Crystal Filter -- described in U.S. Pat. No. 3,564,463 to W.D. Beaver and R.A. Sykes which is assigned to Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey. Such a crystal filter may be comprised of two pairs of aligned metallic electrodes plated on opposite faces of a piezoelectric quartz crystal wafer. Energy supplied to an input electrode pair at or near the shear mode fundamental frequency of the crystal wafer will piezoelectrically vibrate the wafer in the shear mode which will be sensed at an output pair of electrodes. Accordingly, only the frequencies corresponding to the fundamental and overtone frequencies of the crystal wafer and a limited band of frequencies about such frequencies will be transmitted from the input to the output of the filter. All other frequencies will be suppressed to a high degree.
Although there are other factors, the controlling contribution to the fundamental or resonant frequency of such a crystal filter is the thickness of the quartz crystal captured between the electrodes. The thickness of the quartz crystal wafer is inversely proportional to its fundamental frequency. Accordingly, the maximum fundamental frequency of the crystal filter is limited by the state of the quartz crystal lapping art. Presently, quartz crystal can be lapped to yield usable wafers having a fundamental frequency up to approximately 60 MHz. However, such wafers must be lapped to about 0.0011 inch (0.0028 cm) in thickness to attain such frequencies which makes such wafers very difficult to fabricate, delicate, subject to breakage and extremely expensive.
An alternative to using the fundamental frequency of a crystal filter to attain high frequency filtering capabilities (i.e., about 40 MHz) is to pass frequencies at a predetermined one of the overtone frequency bands of the crystal filter. However, this normally requires the use of additional LC filters to suppress input frequencies corresponding to the fundamental frequency and overtones that should not pass through the crystal filter.